The subject matter described herein relates generally to methods and systems for operating an off-shore wind turbine, and more particularly, to methods and systems for operating a control for an off-shore wind turbine.
Generally, a wind turbine includes a turbine that has a rotor that includes a rotatable hub assembly having multiple blades. The blades transform wind energy into a mechanical rotational torque that drives one or more generators via the rotor. The generators may be rotationally coupled to the rotor through a gearbox. The gearbox steps up the inherently low rotational speed of the rotor for the generator to efficiently convert the rotational mechanical energy to electrical energy, which is fed into a utility grid via at least one electrical connection. Gearless direct drive wind turbines also exist. The rotor, generator, gearbox and other components are typically mounted within a housing, or nacelle, that is positioned on top of a base that may be a truss or tubular tower.
Some wind turbine configurations include double-fed induction generators (DFIGs). Such configurations may also include power converters that are used to convert a frequency of generated electrical power to a frequency substantially similar to a utility grid frequency. Moreover, such converters, in conjunction with the DFIG, also transmit electric power between the utility grid and the generator as well as transmit generator excitation power to a wound generator rotor from one of the connections to the electric utility grid connection. Alternatively, some wind turbine configurations include, but are not limited to, alternative types of induction generators, permanent magnet (PM) synchronous generators and electrically-excited synchronous generators and switched reluctance generators. These alternative configurations may also include power converters that are used to convert the frequencies as described above and transmit electrical power between the utility grid and the generator.
Known wind turbines have a plurality of mechanical and electrical components. Each electrical and/or mechanical component may have independent or different operating limitations, such as current, voltage, power, and/or temperature limits, than other components. Moreover, known wind turbines are typically designed and/or assembled with predefined rated power limits. To operate within such rated power limits, the electrical and/or mechanical components may be operated with large margins for the operating limitations. Such operation may result in inefficient wind turbine operation, and the power generation capability of the wind turbine may be underutilized.
Typical wind turbines are operated by a wind turbine control which particularly implements pitch control by rotation of the rotor blades about a pitch axis. That is, these control systems are designed for regulating the rotor speed of the wind turbine by setting the angles of the blades, i.e., pitching the blades, with respect to the airflow. Pitching the blades for decreasing the rotor speed generally results in a decrease of the load acting on some of the components of the wind turbine, such as the blades, the rotor, or the wind tower.
Generally, an increase of the speed of the wind impinging on the rotor blades causes an increase of the rotor speed. Under conditions such as high winds in the area of the wind turbine, the rotor speed may eventually exceed a threshold value corresponding to the maximum allowable speed of the wind turbine (i.e., an overspeed).
At least some known control systems which implement pitch control are designed for monitoring the rotor speed by determining actual values thereof and aerodynamically decreasing the rotor speed by increasing the pitch angle of the blades as soon as the “rated speed” is reached. The rated wind speed is the minimum wind speed at hub height at which a wind turbine's rated power is achieved in the case of steady wind without turbulence. The rated wind speed and the rated power is typically a constant for a wind turbine, and wind turbine manufacturers do normally provide information thereabout.
In this situation, sudden decrease of the rotor speed by pitching the blades may result in a particularly significant increase of the load acting on components of the wind turbine. A significant load increase negatively influences the operating life of the turbine. In at least some known pitch control systems, the pitch control drives the rotor speed back to or below a certain set-point value of the wind turbine.
The increase and posterior decrease of the pitch angle generally results in alternating forces acting on the tower. In some cases, these alternating forces may excite the resonant modes of the tower and lead to a resonant vibration of the tower. Such a resonant vibration of the tower may require shutting down the wind turbine when the vibration exceeds a maximum allowable limit. A shutdown event generally implies a loss of the capacity for generating power by the wind turbine.
Offshore wind turbines are additionally exposed to water conditions impacting the base of the wind turbine. The water conditions can provide additional constant load and may also stimulate the system in a resonance frequency. Undesired oscillations can result. This may lead to damage to or premature aging of the wind turbine.
Accordingly, it is desirable to provide a method and a wind turbine capable of implementing a wind turbine control which avoids high load on the wind turbine components and diminishes the risk of a shutdown of the wind turbine due to an overload state or fatigue of the wind turbine.